Dynamics of excited molecular states

Abstract

The photo-excitation or photo-ionization of a polyatomic molecule is typically accompanied by a strong excitation of the vibrational modes. In particular when a conical intersection of the electronic potential energy surfaces involved lies within or close to the Frank-Condon zone, the nuclear motion becomes very complicated, often chaotic, and the spectra become irregular and dense.An accurate simulation of the dynamics of such excited molecules requires firstly that the multi-dimensional and multi-state potential energy surface – or a reliable model thereof – can be determined. Secondly, the multi-dimensional quantum dynamics have to be solved. This is a very difficult task, because of the high dimensionality of the problem (6 to 30 degrees of freedom, say). The multi-configuration time-dependent Hartree (MCTDH) method has proven to be very useful for the study of such problems. In fact, an accurate treatment of the quantal dynamics of molecules like the allene cation (C3 H+4, 15D), the butatriene cation (C4 H+4, 18D), or the pyrazine molecule (C4N2H4, 24D) in their full dimensionality, is – up to date – only possible with MCTDH. (The acronym n D denotes the dimensionality.)The construction of the vibronic model Hamiltonian and the MCTDH method will be briefly discussed. After this, the excited state dynamics of the butatriene and pyrazine molecules will be discussed.